1. Field of Invention
The present invention relates to testing communication between transmitter-receiver pairs over electrical power transmission lines.
2. Description of Prior Art
In electrical power transmission and distribution systems, carrier relay equipment in the form of transmitter-receiver pairs has been provided at various stations or substations to communicate information, such as whether or not circuit breakers at a particular station are to be tripped when a fault or ground is detected in the system. The operability of these transmitter-receiver pairs and the integrity of their connection over the power transmission lines had to be periodically tested.
One method of testing was to send test technicians to both stations who would operate the carry relay equipment, sending signals between the two stations and verifying receipt of signals from the other station. This clearly was expensive in terms of labor costs.
Two apparatus were proposed in an attempt to solve this problem. Both sent a test signal from a transmitter at a first or master station over the power line to a receiver at a second or slave station for a time interval. At the slave station, a transmitter was activated after receiving the transmitted master station signal, sending a return signal back to the master station. Unless the master station received the return signal in a particular time interval, an alarm was activated. One apparatus used a rotary mechanical drum with rubber cogs which engaged switches to control timing functions. In service usage, dust and other contamination often caused the switches to stick, requiring periodic cleaning of the equipment. Also, the rubber cogs deteriorated with time, becoming brittle and breaking and requiring replacement. The second apparatus used complicated electronics with digital logic circuitry which typically did not stand up to the transient conditions and voltage levels present in power distribution.
U.S. Pat. Nos. 4,044,351 and 3,970,940 tested transmitter-receiver communication over telephone lines and radio waves, respectively. Each was based on detection of time coincidence between the transmitted and received signals. U.S. Pat. No. 4,032,909 tested an alarm system by a test operator tripping a remote sensor and monitoring the response of an alarm with an audio transmitter at a central station. The monitored response was sent by radio waves to a receiver with the test operator to indicate the sensor was in communication with the central station. U.S. Pat. No. 3,991,362 related to a system for preventing tampering with a remote alarm, while U.S. Pat. No. 3,955,183 related to an operability indicator for testing an alarm without causing a false alarm.